Clomiphene is a selective estrogen receptor modulator related to tamoxifen. Clomiphene binds to estrogen receptors and blocks the normal estrogen feedback on the hypothalamus and subsequent negative feedback on the pituitary. This leads to increases in luteinizing hormone (LH) and follicle stimulating hormone (FSH). In men, these increased levels of gonadotropins stimulate the Leydig cells of the testes and result in the production of higher testosterone levels. For example, Tenover et al., J. Clin. Endocrinol. Metab. 64:1103, (1987) and Tenover et al., J. Clin. Endocrinol. Metab. 64:1118 (1987) found increases in FSH, LH in both young and old men after treatment with clomiphene. They also found increases in free and total testosterone in men with young men showing significant increases.
In females, clomiphene is currently approved as a mixture of both cis- and trans-isomers, the cis-isomer being present as about 30% to 50% (Merck Manual) for the induction of ovulation in anovulatory women. The increases in LH and FSH in anovulatory females following administration of clomiphene result in follicular growth and ultimately ovulation. The drug is recommended to be administered for 5 days at a dose of up to 100 mg daily
Ernst et al., J. Pharmaceut. Sci. 65:148 (1976), have shown that clomiphene is a mixture of two geometric isomers which they refer to as cis,-Z-, clomiphene (cis-clomiphene or zuclomiphene) and trans-,E-, clomiphene, (trans-clomiphene or enclomiphene). According to Ernst, et al. trans-clomiphene HCl has a melting point of 149° C.-150.5° C., while cis-clomiphene HO has a melting point of 156.5° C.-158° C. Ernst et al. have also noted that (the trans-isomer) is antiestrogenic (AE) while the cis-isomer is the more potent and more estrogenic form and has also been reported to have anti-estrogenic activity. The authors attribute the effect of the drug on ovulatory activity to both forms stating that the mixture is more effective than trans-clomiphene alone. The trans-isomer aids ovulation at the level of the hypothalamus. The estrogenic isomer cis-clomiphene contributes to enhanced ovulation elsewhere in the physiologic pathway leading to ovulation. The isomers are also reported to have different in vivo half-life. The cis isomer has been reported to leave residual blood levels for in excess of one month following a single dose.
Clomiphene has been associated with numerous side effects including: blurred vision, abdominal discomfort, gynecomastia, testicular tumors, vasomotor flushes, nausea, and headaches. Furthermore, other studies suggest that clomiphene possesses both genotoxic and tumor enhancement effects. The net outcome of these observations is that clomiphene in its current format, having between 30% and 50% of the cis isomer, would be unacceptable for chronic therapy in men for the treatment of testosterone deficiency.
Oral administration of trans-isomer of clomiphene (trans-clomiphene or enclomiphene) has been demonstrated to be effective in the treatment of a panoply of disorders ranging from secondary hypogonadism in males to induction of ovulation in anovulatory females. However, first pass metabolism of the drug by the liver requires relatively high doses to be administered orally to achieve therapeutic effect. A significant advance in the art would result if active metabolites of trans-clomiphene were discovered.